Assembly of tension leg platforms ("TLP's") is a traditional problem in offshore exploration and production areas, one cause of which is the resonance action of waves on the float columns. As shown in FIGS. 1a1 and 1a2, a standing wave may act upon the TLP to generate resonant heave (vertical) motion in the TLP. When the trough of the wave passes the TLP, the sea provides a smaller buoyancy force because less water is displaced by the column. When the crest of the wave passes the TLP, a larger buoyancy force is provided because more water is displaced. The heave motion is detrimental to TLPs because they are secured to the sea floor by tendons and heave resonance causes the tendons to fail. Also, the action of the waves against the side of the column generate vibrations in the TLP system which, if they occur at a harmonic frequency of the TLP system, may cause the tendons to fail. Thus, TLP need to have effectively transparent cross-sections in the wave zone after being secured to the sea floor.
Thus, prior TLP configurations comprise a relatively transparent structures in the wave zone to reduce the effects of wave loading. The traditional TLP configuration comprises a horizontal submerged float that is connected to the platform by vertical supports. Monopods, such at that disclosed in Monopod TLP Improves Deepwater Economics, PETROLEUM ENGINEER INTERNATIONAL (January 1993), incorporated herein by reference, comprise a central monopod support attached to a plurality of submerged floats, such as corner columns. Other platform structures have been proposed which comprise a monopod, but instead of corner columns, they comprise a single column, as shown in FIGS. 1a1 and 1a2, from which the monopod extends. Hove while prior TLPs provide relatively transparent structures in the wave zone, they are unstable prior to attachment to the sea floor. These TLPs typically require assembly of the main production platform after being transported to the operation site. The TLP instability makes the platform assembly a difficult and costly procedure requiring a large derrick barge to stabilize the TLP. Disassembly is likewise difficult so that the TLPs are practically immobile so that they cannot be transported from one production site to another without reducing the TLP's topside weight.
Therefore, there is a need for a TLP which provides greater stability during assembly and transportation, without sacrificing a transparent wave zone structure which is required after the TLP is secured to the sea floor.